#define _CRT_SECURE_NO_WARNINGS 1

class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        vector<int> dp(n + 1);
        dp[0] = 1;
        dp[1] = s[0] != '0' ? 1 : 0;
        if (s[0] == '0')
            return dp[1];

        for (int i = 2; i <= n; ++i)
        {
            if (s[i - 1] != '0')
                dp[i] += dp[i - 1];
            int lr = ((s[i - 2] - '0') * 10) + (s[i - 1] - '0');
            if (lr >= 10 && lr <= 26)
                dp[i] += dp[i - 2];

            if (dp[i] == 0)
                return 0;
        }

        return dp[n];
    }
};